The invention relates to imaging three-dimensional (3D) volume data. In particular, the invention relates to imaging of the 3D volume data with respect to a desired viewpoint and view direction.
The process of calculating two-dimensional (2D) images of 3D objects is often referred to as volume rendering. Volume rendering finds applications in many fields. One such field is the rendering of medical volume data resulting, for example, from the scanning of the human or animal body using computed tomography (CT) and other X-ray scanners, nuclear magnetic resonance scanners and ultrasound scanners, to name but a few examples.
The volume data generated by modern scanning equipment can be very detailed and complex to interpret. A physician may wish to render the data using different view directions and from different positions with respect to the scanned object in order to be able to analyse the scanned object and to detect, for example, abnormalities.
Various techniques are known for rendering 3D volume data to provide 2D images. Some of these are described by Lacroute [1]. These techniques commonly include projecting the volume data onto an image plane perpendicular to a desired view direction. This is often achieved by applying a coordinate transform to the volume data (to effect a change in view direction) and a projection of the transformed data along a line-of-sight onto the view plane (to form the 2D image). Coordinate transforms are generally made by applying a so-called view transform matrix. The projection of the transformed volume data can be made in a number of ways depending on the desired appearance of the final image.
In some rendering algorithms, the view transform matrix is factorised into two components. One such technique is known as shear-warp factorisation. Examples of this technique are described by Lacroute and in U.S. Pat. No. 5,787,889 [2]. In this approach, a view transform matrix is factorised into a 3D shear transform that is parallel to slices of a reference volume and a 2D warp transform to produce a projection of the sheared volume. This technique allows for faster and more efficient volume rendering algorithms.
Volume rendering techniques, such as applied to slab multi-planar reformatting (MPR) (sometimes referred to as MPR with thickness, or thick MPR), often lead to undesirable artefacts appearing in resulting 2D images. These artefacts are visually distracting and can hinder the interpretation of the images. In some situations the artefacts could be mistaken for real features of the volume data or in other cases could obscure real features of the data. Artefacts can also have a deleterious effect on subsequent image processing. For example, the accuracy of edge-detection algorithms is often very sensitive to the presence of image artefacts.
FIG. 1A schematically shows a medical image having an artefact which occurs in some situations with known MPR rendering techniques. The image of FIG. 1A is derived from an MPR slab through the torso of a human patient. A section through the patient's spine and ribcage can be seen. The ribcage encloses a number of the patient's organs. The patient's liver can be seen towards the upper-left region of the image. In this example, the artefact appears as a hatch pattern across the extent of the liver. This hatch pattern is undesirable as it is visually distracting and can obscure structures in the volume data which have a size comparable to the scale of the hatching. Some types of tumour can have a granular structure which can be obscured by the hatching, for example.
The image corresponds to a display of 2D image data generated from 3D volume data (i.e. a volume data set). In this example, the volume data are CT volume data derived from an X-ray CT scan of the patient. However, a similar artefact is seen in images derived from volume data from other imaging modalities. The volume data comprise a plurality of voxels arranged in a 3D grid. Each voxel has a voxel value associated with it. The voxel values represent measurements of a physical parameter of the patient. In this example, the voxel values represent an opacity of the patient's tissue to X-rays, and is measured in Hounsfield units (HU). This is very closely correlated with density (mass per unit volume). The volume data therefore correspond to the variation of density throughout the imaged part of the patient's torso.
The volume data are aligned with three orthogonal axes I, J and K having a common origin at one corner of the volume data. However, it will be appreciated that this choice of origin is arbitrary. These axes define a volume space. A volume-space coordinate system is used to identify the location of each voxel in volume space. The volume-space coordinate system has unit (or basis) vectors i, j and k which are aligned with respective ones of the orthogonal axes I, J and K. The unit vectors i, j and k are defined such that the voxels are of unit length along each of the axes in volume space. That is to say, the separation between voxels (i.e. the distance between their centres) along each axis is unity.
The 2D image-data comprise a plurality of image-pixels arranged in a 2D grid. Although the image itself is 2D, it is helpful to define a 3D view space containing the image. View space is defined by three orthogonal axes X, Y, Z having a common origin at one corner of the image. Again, the choice of origin is arbitrary. The X- and Y-axes are in the plane of the image (the image plane) and are aligned with the 2D grid of image pixels. The Z-axis is aligned parallel with the view direction (i.e. perpendicular to the image plane). A view-space coordinate system is defined to identify the location of each voxel and each image pixel in view space. The view-space coordinate system has unit, or basis, vectors x and y in the image plane and z along the view direction. The unit vectors x and y are defined such that the image pixels are of unit length along each of the axes in view space.
The image shown in FIG. 1A is generated from the volume data using a conventional slab multi-planar reformatting (MPR) technique. This technique involves applying a view transform matrix to effect a coordinate transform for the volume data from volume space to view space. The volume data transformed to view space are often referred to as MPR data, or as an MPR slab. The MPR slab comprises a series of MPR slices which are aligned parallel to the image plane and disposed at different positions along the Z-axis. It will be appreciated that often only a subset of the volume data, for example volume data extending over a selected region in volume space, will be processed. For example, a user may identify a region containing an organ of interest with only volume data from this region being subsequently rendered.
The 2D image is formed by projecting (collapsing) the MPR slab along the view direction onto the image plane. This is done according to a projection algorithm. The projection algorithm used in any particular case will depend on the desired appearance of the final image. One commonly used projection algorithm, and the one used for the image shown in FIG. 1A, is to determine for each image-pixel the maximum voxel value seen in the MPR slab along the Z-axis for the XY-coordinate corresponding to that image pixel. This is known as maximum intensity projection. Maximum intensity projection is a type of ray casting. In effect, for each pixel in the image, an imaginary ray is cast through the volume data parallel to the view direction. The image data for each pixel is then taken to be the maximum voxel value encountered by the ray as it traverses the MPR slab. Another projection algorithm known as minimum intensity projection is similar, but uses the minimum voxel value encountered by rays traversing the MPR slab for the image data instead of the maximum.
It will be appreciated that in some cases, only voxels having a voxel value in a selected range, or “window” will be of interest. For example, to reveal soft tissue on a CT scan, only voxel values in the range −200 to 500 HU may be of interest. To achieve such a view, a maximum or minimum intensity projection MPR image is typically calculated as described above, and subsequently the image is post-processed to enhance the contrast of voxel values in the desired range and suppress contrast outside that range.
The hatch artefact seen in the image shown in FIG. 1A has been found to be more apparent in some situations than in others. In addition, the nature of the artefact, i.e. the scale (periodicity) and intensity of the hatching has been found to be sensitive to view point and view direction.
FIGS. 1B, 1C and 1D schematically show images derived from the same volume data as that used for the image of FIG. 1A. However, these images have been rendered from different directions.
FIG. 1B corresponds to a zoom of approximately 110% compared to the image of FIG. 1A. The characteristic periodicity of the hatch artefact seen on the liver in FIG. 1B can be seen to have reduced by several times.
FIG. 1C corresponds to a rotation of view-space around the view direction by about 5° compared to the image shown of FIG. 1A. The hatch artefact is also apparent across the liver shown in FIG. 1C.
FIG. 1D corresponds to a rotation of view-space around the view direction by about 7°, i.e. a relative rotation of around 2° compared to the image shown in FIG. 1C. For these viewing conditions, the characteristic periodicity of the hatch artefact across the liver seen in FIG. 1D is significantly smaller than that seen in FIG. 1C.
This sensitivity of the artefact's appearance to changes in viewing conditions exacerbates its distracting effects. This is particularly so where, for example, a user wishes to animate a series of images which correspond to different view directions, or to continuously rotate, zoom or pan an image.
The artefact has been found to be most apparent when one or more of the following conditions apply:                the volume data includes significant variations (real or noise) occurring on a spatial scale comparable to, or smaller than, the voxel size;        only a narrow window of voxel values are to be included in the projection, for example to isolate soft tissue from surrounding tissues;        the MPR slab has a significant extent along the view direction        maximum (or minimum) intensity projection is used;        the image plane (parallel to planes of the MPR slices in the MPR slab) is parallel, or closely parallel, to a plane containing a pair of axes of the volume-space coordinate system;        the image is scaled (zoomed) by a non-integer factor; and/or the rotation of view-space around the view direction is such that rows of voxels in the volume data do not project onto rows of pixels in the output image (i.e. the rotation is not exactly 0°, 90°, 180° or 270°).        